The claimed subject matter relates to the switchover of mobile terminal devices from a first radio network of a mobile radio system to a second radio network within the scope of a CS (circuit-switched) fallback mechanism. The claimed subject matter relates to a method for carrying out such a mobile terminal device switchover, to a base station of a radio network that can be used to carry out the switchover as well as to a mobile terminal device that can be used to carry out the switchover.
The term, 4G (or Long-Term Evolution (LTE) refers to mobile radio networks that have been standardized by the 3rd Generation Partnership Project (3GPP). In the development of these networks, the transmission of data is in the foreground, ahead of establishing voice calls. In contrast to the mobile radio networks of the previous generation, 4G networks make use of packet-switched (PS) connections between the networks and the associated mobile terminal devices. There are no provisions for circuit-switched connections of the type provided in 2G networks according to the Global System for Mobile Communications (GSM) standard, and in 3G networks according to the Universal Telecommunications (UMTS) standard, for voice communication. The 4G networks establish voice calls by using a packet-switched connection as a service that is made available to the 4G networks by the IP multimedia subsystem (IMS).
In the initial phase of operation of 4G networks, the services of the IMS may merely be available to a limited extent or not at all. However, in order to give mobile terminal device users the ability to make voice calls, a circuit-switched (CS) fallback mechanism is provided, which is described in the 3GPP specification TS 23.272. Here, in order for a mobile terminal device to establish a voice call that is made via a circuit-switched connection, it switches over from the 4G network in a defined sequence to a 2G or 3G network (or to the CS domain of such a network) that is spatially superimposed over the 4G network. The network is switched over from the 4G radio network to the 2G or 3G radio network and, after the radio network switchover, the core network components of the 2G or 3G network that function on the basis of a circuit-switched mechanism.
In the case of an outgoing (mobile originated—(MO)) call, the mobile terminal device first informs the 4G network that it would like to make a voice call by the CS fallback. After a prompt by the 4G network to this effect, the terminal device switches over to a cell of a 2G or 3G network that it has selected itself or that is prescribed by the 4G network, and the terminal device then logs on to this network. The voice call is then established. In the case of an incoming (mobile terminated—(MT)) call, the terminal device is first informed by the 4G network about the call. After an acknowledgement and after a prompt by the 4G network to this effect, the terminal device then switches over to a cell of a 2G or 3G network that it has selected itself or that is prescribed by the 4G network. After the log-on to this network, the terminal device signals its availability to accept the call. Then the call is established in the 2G or 3G network on the basis of circuit-switched mechanism. The network is switched over from the 4G network to the 2G or 3G network and, after the radio network has switched over, the circuit-switched core network components of the 2G or 3G network are used.
Currently, the implementation of the CS fallback mechanism entails the problem that the base station of the radio network via which the circuit-switched connection is established, for example, the base station of the 2G or 3G network, does not receive any information about the fact that the access by a terminal device is being carried out on the basis of a radio network switchover within the scope of the CS fallback mechanism. As a result, for instance, it is not possible to determine a failure rate of radio network switchovers carried out within the scope of a CS fallback mechanism and, if a high failure rate is found, to take appropriate measures to prevent such failures.